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Apache Commons Lang, a package of Java utility classes for the classes that are in java.lang's hierarchy, or are considered to be so standard as to justify existence in java.lang.

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/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 * 
 */

package org.apache.commons.lang3.builder;

// adapted from org.apache.axis.utils.IDKey

/**
 * Wrap an identity key (System.identityHashCode()) 
 * so that an object can only be equal() to itself.
 * 
 * This is necessary to disambiguate the occasional duplicate
 * identityHashCodes that can occur.
 * 
 */ 
final class IDKey {
        private final Object value;
        private final int id;

        /**
         * Constructor for IDKey
         * @param _value The value
         */ 
        public IDKey(final Object _value) {
            // This is the Object hashcode 
            id = System.identityHashCode(_value);  
            // There have been some cases (LANG-459) that return the 
            // same identity hash code for different objects.  So 
            // the value is also added to disambiguate these cases.
            value = _value;
        }

        /**
         * returns hashcode - i.e. the system identity hashcode.
         * @return the hashcode
         */ 
        @Override
        public int hashCode() {
           return id;
        }

        /**
         * checks if instances are equal
         * @param other The other object to compare to
         * @return if the instances are for the same object
         */ 
        @Override
        public boolean equals(final Object other) {
            if (!(other instanceof IDKey)) {
                return false;
            }
            final IDKey idKey = (IDKey) other;
            if (id != idKey.id) {
                return false;
            }
            // Note that identity equals is used.
            return value == idKey.value;
         }
}